Since the motherboard of computers has been changed from AT specifications to ATX specifications, the structure of power supply that provides electric power also is changed. One of the notable features is that output of the power supply is divided into primary output power and standby power that are supplied respectively by a primary power system and a standby power system of the power supply. The standby power system receives input power from an input power source and starts. The input power is converted to the standby power which is provided to form a sleeping mode in regular conditions. Such a state provides a remote start function required in the ATX specifications. When users push a start button or order the computer to start from a remote site, the standby power provides power to start the primary power system. Then the primary power system generates the primary output power to supply electronic equipments for operation. Referring to FIG. 1, a conventional power supply includes a rectification unit 1, a power factor correction unit 2, a primary power system 3 and a standby power system 4. The rectification unit 1 receives and rectifies input power 91, and the power factor correction unit 2 boosts the voltage and regulates the phase of the rectified power. On the other hand, the standby power system 4 is connected to a control unit 41 to receive at least one electric signal. The standby power system 4, according to parameters or power supplied by the electric signal, converts the input power 91 to standby power 94 which becomes the power source of the power supply and electronic equipments in standby conditions. A remote start/stop signal source 93 (such as a button depressed and triggered by users) is provided to generate a remote start/stop signal to activate the primary power system 3 to generate output power 92. Thus the standby power 94 provides the power needed to start the primary power system 3. The standby power system 4 also continuously generates the standby power 94 in the standby conditions. As a result, loss continuously takes place on the standby power system 4. The above-mentioned is a starting process of a conventional power supply. With the output watts of the power supply increased constantly, output watts of the standby power 94 also increase to such an extent almost capable to support a small electronic device, such as charging a handset inserted in a USB slot, running a small fan inserted in a USB slot or the like. Thus utilization of the standby power 94 is expanded. However, power loss of the standby power 94 also increases due to the greater output watts. When the primary power system 3 is shut down, the standby power system 4 continuously gets the input power and functions continuously. As the loss generated by the standby power system 4 is directly affected by its output, the more output delivered by the standby power system 4, the more unnecessary loss occurs in the standby conditions. This also results in a higher temperature of the equipment in the standby conditions. As the cooling fan usually is turned off when the primary power system 3 is shut down, cooling of the equipment in the standby conditions is adversely affected. There are still rooms for improvement.